Many data processing systems require a large amount of data storage, for use in efficiently accessing, modifying, and re-storing data. Data storage is typically separated into several different levels, each level exhibiting a different data access time or data storage cost. A first, or highest level of data storage involves electronic memory, usually dynamic or static random access memory (DRAM or SRAM). Electronic memories take the form of semiconductor integrated circuits where millions of bytes of data can be stored on each circuit, with access to such bytes of data measured in nanoseconds. The electronic memory provides the fastest access to data since access is entirely electronic.
In certain data processing applications, a plurality of interconnected computer systems, sometimes referred to as “host computers,” provide information to a plurality of data storage and retrieval systems. It would be desirable to select a captain control node from among the host computers to coordinate the operations of those host computers.
A second level of data storage usually involves direct access storage devices (DASD). DASD storage, for example, includes magnetic and/or optical disks. Data bits are stored as micrometer-sized magnetically or optically altered spots on a disk surface, representing the “ones” and “zeros” that comprise the binary value of the data bits. Magnetic DASD includes one or more disks that are coated with remnant magnetic material. The disks are rotatably mounted within a protected environment. Each disk is divided into many concentric tracks, or closely spaced circles. The data is stored serially, bit by bit, along each track.
In certain data processing applications, a plurality of host computers provides information to a plurality of interconnected data storage and retrieval systems. It would be desirable to select on a performance basis a captain control node from among the plurality of data storage and retrieval systems to coordinate the operation of those systems.
Having a backup data copy is mandatory for many businesses for which data loss would be catastrophic. In addition, protection is required for recovering data if the entire system or even the site is destroyed by a disaster such as an earthquake, fire, explosion, hurricane, etc.
Disaster recovery requires that the secondary copy of data be stored at a location remote from the primary data. The secondary site must not only be sufficiently remote from the primary site, but must also be able to backup primary data in real time. The secondary site needs to backup primary data in real time as the primary data is updated, with some minimal delay. A difficult task required of the secondary site is that the secondary data must be “order consistent,” that is, secondary data is copied in the same sequential order as the primary data (sequential consistency) which requires substantial system considerations. Sequential consistency is complicated by the existence of multiple storage controllers each controlling multiple DASDs in a data processing system. Without sequential consistency, secondary data inconsistent with primary data would result, thus corrupting disaster recovery.
In certain data processing applications, a plurality of interconnected data storage and retrieval systems provides data to a plurality of interconnected primary backup appliances. The primary backup appliances form what are sometimes referred to as consistent transactions sets, and periodically provide those consistent transactions sets to a remote site for backup storage. It is desirable to select on a performance basis a captain control node from the plurality of primary backup appliances to coordinate the operation of those backup appliances.